A hepatobiliary iminodiacetic acid scan, commonly known as a HIDA scan, is a specialized medical imaging procedure that provides a functional assessment of your upper digestive system. It uses a small amount of a radioactive tracer to evaluate the function of the liver, gallbladder, and bile ducts. By tracking the flow of this tracer, doctors can diagnose issues such as inflammation, blockages, or leaks within the biliary system. Understanding the preparation instructions is important for obtaining accurate results.
The Required Fasting Period
To ensure the HIDA scan is successful, you must fast for a specific period before the procedure. The typical mandatory fasting window is at least four hours prior to the scan, but usually not more than 12 to 15 hours. Fasting for too long can also negatively affect the test results.
During this period, you should not consume anything that could stimulate your gallbladder to contract and empty. This includes coffee, juice, milk, gum, or mints, as even these items can initiate the digestive process. Plain water is generally permitted and often encouraged to maintain proper hydration. Always confirm the specific duration and exact restrictions with your ordering physician or the imaging facility.
Why Preparation is Essential
The requirement to fast is directly related to the normal physiology of the gallbladder, which serves as a reservoir for bile produced by the liver. When you eat, particularly foods containing fat, your body releases a hormone that signals the gallbladder to contract forcefully. This contraction releases bile into the small intestine to aid in digestion.
For the HIDA scan to work, the radioactive tracer—which the liver excretes into the bile—must have time to travel and accumulate inside the gallbladder. If you eat before the scan, the gallbladder will empty prematurely, preventing the tracer from filling the organ. This premature emptying can lead to a non-diagnostic result or a false-positive finding.
It is also important to discuss any medications you take with your doctor, as some, particularly opiate-based pain medications, can interfere with the results by affecting the bile ducts. Most other necessary prescriptions can be taken with a small sip of water during the fasting window. Following these instructions ensures the gallbladder is relaxed and ready to accept the tracer.
What to Expect During the Scan
The HIDA scan procedure begins with a technologist injecting a small amount of the radioactive tracer into a vein in your arm. This tracer is non-toxic and emits gamma rays, which are tracked by a specialized camera positioned over your abdomen. The camera captures a series of images as the tracer moves from the bloodstream through the liver, into the bile ducts, and eventually into the gallbladder and small intestine.
The entire imaging process typically takes between one and four hours, depending on how quickly the tracer progresses through your biliary system. If the initial images are inconclusive or if the physician needs to assess the gallbladder’s ability to empty, a medication called sincalide (a synthetic version of the hormone cholecystokinin) may be administered intravenously. This injection stimulates the gallbladder to contract, allowing doctors to calculate its ejection fraction, which is a measure of its function.
When the sincalide is injected, it can often induce mild abdominal cramping or discomfort. Once the necessary images are captured, you will be able to leave and resume your normal activities. The images are then reviewed by a radiologist who provides a detailed report to your referring physician.